As a device that is mounted on a vehicle so as to reduce vibration occurring in a vehicle, PTL 1 discloses a drive system rotation fluctuation reducing device that reduces rotation fluctuation of a drive system including an internal combustion engine, a transmission shaft transmitting an output torque of the internal combustion engine to a drive shaft of a vehicle, and a transmission disposed in the transmission shaft. Such a drive system rotation fluctuation reducing device includes variation means for causing inertia of the transmission shaft to vary and control means for controlling the variation means. In the drive system rotation fluctuation reducing device, a damper that absorbs fluctuation of the output torque is disposed closer to the internal combustion engine than the transmission of the transmission shaft, and the variation means causes the inertia of the transmission shaft, closer to the transmission than the damper, to vary. Accordingly, the drive system rotation fluctuation reducing device can increase the inertia of the transmission shaft while suppressing a decrease in the mode frequency of a primary eigenvalue in a torsional vibration mode of the drive system, by increasing the inertia of the transmission shaft closer to the transmission than the damper. As a result, the drive system rotation fluctuation reducing device can reduce the rotation fluctuation of the drive system while suppressing a decrease in vehicle responsiveness.